The present invention generally relates to wireless communications receivers, and more particularly relates to techniques for improving receiver processing using probability metrics associated with demodulated soft bits.
In today's advanced wireless systems, both the uplink (mobile terminal-to-base station communications) and downlink (base station-to-mobile terminal communications are subject to various sources of interference, including, for example, intra-cell interference arising from a lack of complete orthogonality between user signals within a wireless system cell, inter-cell interference arising from signals intended for users or originating from users in other cells, and thermal noise. To combat these interference sources, interference cancellation techniques are increasingly being deployed.
One category of interference cancellation techniques is known as decoder interference cancellation (IC) or post-decoder interference cancellation. The general idea behind decoder interference cancellation is that a signal generated using decoder output from a first decoding attempt is subtracted from the input signal before a second decoding attempt. The decoder output from the first decoding attempt could relate to a signal that is an ultimate target of the demodulation/detection process, or it might represent decoded bits of an unwanted signal, for the purpose of cancellation.
FIG. 1 illustrates an example of an interference cancelling receiver 100 that uses output from the decoder to perform interference cancellation. This receiver system is sometimes referred to as a Turbo interference cancellation receiver. In the figure, a so-called RAKE receiver is shown, which indicates a Wideband Code-Division Multiple Access (W-CDMA) application of decoder interference cancellation. However, the general structure works equally well for Long-Term Evolution (LTE) applications, which are based on Orthogonal Frequency-Division Multiple Access (OFDMA), if the RAKE components are replaced with Fast-Fourier-Transform (FFT)-based receiver components.
As can be seen in FIG. 1, the output of the decoder 140 produces log-likelihood ratios (LLR), which are essentially estimated probabilities that the corresponding decoded information bits should be set to one or zero. For interference cancellation purposes, the LLRs are used by soft mapper 150 to generate the symbol values that were most likely to have been transmitted by another node, such as a base station when the receiver is in a wireless terminal. An estimate of the received signal corresponding to these symbol values is produced by signal regenerator 160, which applies the same modulation and scrambling that was removed from the signal by RAKE despreader 110 and demapper 130 in the first decoding iteration. Then, the regenerated signal from signal regenerator 160 is subtracted from the original input signal, using subtracter 105, to produce an interference-reduced signal. At the output of the RAKE despreader 110, the contribution to the despread signal from the original received signal (for a given code) is denoted yk(i) and the contribution from the subtracted signal is denoted as {tilde over (y)}k(i), where i is the symbol index and k the channelization code.
An equalizer, illustrated as a G-RAKE combiner 120 in FIG. 1, applies equalizer weights to the despread signal to reduce the effects of multipath propagation. The equalizer weights, or G-RAKE weights, since the weights are applied after the RAKE, are denoted w.
Before demapping of the interference-reduced and equalized signal, that is, before conversion from soft symbols to soft bits, the earlier subtracted signal is added back, in the event that the subtracted signal is the signal we want to decode. In FIG. 1 this is denoted by c·{tilde over (s)}k(i), which is added to the G-RAKE combiner output by adder 125. Here, the scaling constant c is a function of the equalizer weights and the net channel estimates. It should be noted that if the subtracted sequence represents the decoded symbols from an interfering cell, the subtracted signal is not added back prior to the demapping by demapper 130. Add back is only done the event that the signal that we are interested in demodulating has been subtracted.
In either case, after demapping by demapper 130, decoding is performed by decoder 140, which produces a new (and improved) set of probabilities (LLR) for the transmitted bits. The above procedure could be repeated as many times as desired, subject to limitations on processing power available in the receiver, limitations on latency, etc. Of course, this iterative process might also be terminated when the remaining errors in the decoded bits fall below a target level.
Intra-cell and inter-cell interference have a fundamental impact on the maximum throughput of a wireless communication system. While previously described interference cancellation schemes help, further improvements are needed.